Process for developing electrostatic images and apparatus therefor



R E. HAYFORD PROCESS FOR DEVELOPING ELECTROSTATIC 2 Sheets-Sheet 1 Nov.4, 1958 IMAGES AND APPARATUS THEREFOR Filed Jan. 11, 1956 IMAGE POWDERPOWDER POWDER TRANSFER CLOUD CLQUD CLOUD AND/OR GENERATOR CHARGERDEVELOPMENT FIXATION lMAGE FQRMATION 5 PLATE SENSITIZATlO 12 as .Q 85 212 A \M w \jo 38 gz-q VACUUM E COLLECTING 50x AlR SUPPLY INVENTOR.RICHARD E. HAYFORD Nov. 4, 1958 R E. HAYFORD 2,859,129

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RICHARD E. HAYFORD BYF i United States Patent O PROCESS FOR DEVELOPINGELECTROSTATIC IMAGES AND APPARATUS THEREFOR Richard E. Hayford,Pittsford, N. Y., assignor to Haloid lenllx Inc., Rochester, N. Y., acorporation of New 7 Application January 11, 1956, Serial No. 558,459

11 Claims. (Cl. 11717.5)

This invention relates to a method and apparatus for the development ofelectrostatic latent images.

In'xerography it is usual to form an electrostatic latent image on asurface. One method of doing this is to charge a photoconductiveinsulating surface and then dissipate the charge selectively by exposureto a pattern of activating radiation. Other means of formingelectrostatic latent images are set forth in U. S. 2,647,464 to James P.Ebert. Whether formed by these means or any other, the resultingelectrostatic charge pattern is conventionally utilized by thedeposition of an electroscopic material thereon through electrostaticattraction whereby there is formed a visible image of electroscopicparticles corresponding to the electrostatic latent image.Alternatively, the electrostatic charge pattern may be transferred to aninsulating film and the electroscopic particles deposited thereon toform the visible image. In any case, this visible image, in turn, may betransferred to a second surface to form a Xerographic print.

The process of depositing the electroscopic powder on the electrostaticimage to render the electrostatic image visible is called thedevelopment step and is one of the most critical steps of the entireprocess. It is evident that.

no picture can be better than the development step permits. About thecoarsest type of image reproduced by a xerographic process requires aresolution of at least about 50 lines per inch. Commercial line-copyingXerographic machines generally have a resolution power of about 125 to250 lines per inch. The process used in obtaining this resolution is setforth in U. S. 2,618,552 and involves the use of a finely-dividedcolored material called a toner deposited on a slightly morecoarsely-divided material called a carrier. This two-component developeris cascaded across the electrostatic image areas. The frictional contactbetween the toner and the carrier charges the particles due totriboelectric contact. Depending on the relative positions of the tonerand carrier in the triboelectric series it is possible to place either apositive or negative charge on the toner.

In continuous tone development where resolutions of about 1200 or morelines per inch are often desired, it has been found impossible to obtainthis high quality of reproduction using such a system. Accordingly, asystem known as powder cloud development is preferred. In this system acloud of colored marking particles is generated as described hereafter,charged, and then contacted with the electrostatic latent image in thedevelopment step.

Various means of charging the marking particles have been used. Thus,the two-component developer used in cascade development may be agitatedin a closed container to charge the particles triboelectrically and thensubjected to an air stream to separate the toner from the carrierentraining the toner in the air stream to thereby form a cloud ofcolored marking particles. This system suflfers from several defectssuch as lack of uniformity,

presence of agglomerates rendering a developed print excessively grainy,and the difficulty of maintaining output for appreciable periods oftime.

Positioning a corona in the stream of entrained marking particles hasalso been tried but sufiers from ditficulties such as inability toimpart adequate charge at the high flow rates desired, deflection of thepowder stream due to corona wind and the high cost of the separate highvoltage power supply rendering the equipment bulky and expensive.

The system which has proven most effective in practice has been to passthe entrained developer particles through a narrow tube in turbulentflow whereby the particles are in repeated contact with the Walls of thetube. Thus, the powder cloud is passed through a capillary whereby it ischarged through triboelectric contact with the walls of the capillary.This eliminates the necessity for a separate power supply and makespossible small, compact and highly effective charging of the powdercloud. A drawback of capillary charging has been that the material mostsuitable for powder cloud development, namely finelydivided charcoal,was charged negatively when used in this device. Even constructingcapillaries from materials imparting a positive charge to resin tonerswhen used as carriersin the two-component development process was noteffective to positively charge finely-divided charcoal. For severalyears there has been a demand for a material having this property. Thus,zinc oxide binder plates, such as described in U. S. patent applicationSerial No. 311,546, filed September 25, 1952, by Middleton and Reynolds,are charged negatively to obtain positive prints. Because of this zincoxide plates have been developed by either cascade or with a magneticbrush with consequent limitations on the quality of reproductionobtained. Although selenium plates are generally charged positively, inseveral processes for obtaining positive prints from negative originals:i. e. reversal development, :a negatively charged image is formed on thesurface of the selenium. Hence, there has long been a need for amaterial which could accept electrons from finely-divided charcoalrather than merely donate electrons to it.

Applicant has now found that if a capillary is constructed from amaterial selected from a special group of poorly conducting orsemiconducting minerals, it is possible to impart a positive charge tofinely-divided charcoal while at the same time avoiding the problem ofexcessive sparking or deposition on the walls of the capillary whichmight be expected from relatively non-conductive properties of theseunusual materials.

In the drawings:

Fig. 1 is a block diagram showing the position of the development stepin an over-all development process which results in a visible image;

Fig. 2 is a semi-diagrammatic illustration of apparatus according to oneembodiment of the invention;

Fig. 3 is a side elevation of a developing zone with an image-bearingmember thereon, according to one embodiment of the invention;

Fig. 4 is an end elevation in cross section of the structure shown inFig. 3 with the image-bearing member removed therefrom;

Fig. 5 is a side elevation of a developing zone with an image-bearingmember thereon, according to one embodiment of the invention; and

Fig. 6 is a fragmentary end elevation in cross section of the structureshown in Fig. 5.

As shown in Fig. 1 the general xerographic process involves theformation of an electrostatic latent image. This is generally, althoughnot always, preceded by a treatment to sensitize, i. e. electricallycharge, the surface on which the electrostatic image is to be formed.The electrostatic latent image, to be useful, must then be renderedvisible which is done in a development step. This is accomplished bydepositing electroscopic particles either on the surface on which theimage was formed or on an insulating surface to which the electrostaticlatent image has been transferred. The method shown in Fig. 1 for doingthis is termed powder cloud development. In this process a powder cloudis generated, charged and then contacted with the electrostatic latentimage in the development step; The visible image so produced may be usedas such, permanently-affixed to the plate or may be transferred toanother material as a sheet of paper or plastic as is well known tothose skilled in the xerographic art.

The instant invention relates to a process which represents asubstantial improvement in the art of powder cloud development. Theprocess of the instant invention makes possible compact and highlyeffective charging of finely-divided charcoal to a positive polarity. Ingeneral, the present invention accomplishes this object by flowing thefinely-divided charcoal suspended in a gas stream through a narrow tubeof soapstone, montmorillonite, halloysite, bentonite, spodumene, redsandstone, pyrophyllite, oligoclase, gray sandstone or flagstone inturbulent flow. The process of the invention and apparatus for itsaccomplishment will now be set forth in more detail.

Referring to Fig. 2 there is illustrated semi-diagrammatically anapparatus suitable for use in the instant invention. According to thisembodiment an air or gas supply 18 is provided from which a tube orconduit 26 leads to an air tight housing 27. Within this housing arepositioned reels 28 carrying a powder impregnated ribbon 30. The reels28 are so mounted that the ribbon 30 passes over guides 29 which leadthe ribbon past nozzle 39 of tube 20. The reels are spring or motordriven in order to move the ribbon at a controlled rate. In practice thenozzle 39 and the ribbon 30 are relatively close together. The airsupply 18 maintains the housing 27 at a pressure greater than ambientwhereby a jet of gas is forced through the nozzle 39 hearing with it asupply of powder-in-gas suspension from the powder impregnated ribbon 30into the tube 20. The gas suspension of powder particles passes throughtube into manifold 14 where it spreads out to pass through jets 13 intothe entrance chamber 12. The liner 9 of tube 20, the nozzle 39, themanifold 14 and the jets 13 are constructed of soapstone jacketed on theoutside with conductive material 11 which in turn is grounded. Asuitable material for the ribbon is napped cotton flannel.

The air supply 18 also supplies air through tube or conduit 19 to jets31 which feed additional air into the entrance chamber 12 where itcombines with the powder cloud. The powder cloud passes through entrancechamber 12 into development zone 25 formed by positioning a xerographicplate 8 consisting of a conductive backing 23 having coated thereon aphotoconductive insulating layer 24. This plate rests on insulatingspacers 37. Means are provided to ground the conductive backing of thexerographic plate.

The development electrode 10 is positioned at a distance of no more thanabout /s inch from the photoconductive insulating layer 24 and,desirably, for good photographic quality of the developed image, no morethan inch. The development electrode 10 is insulated from the rest ofthe assembly by means of insulating spacers 38. The insulating spacers37 position the xerographic plate 8 at the correct distance from thedevelopment electrode and act as dust seals at the ends of thexerographic plates. Similarly, insulating spacers, not

shown, serve the same purpose for the sides of the assembly.

Means are provided for applying an electric potential difference betweenthe backing member 23 of the xerographic plate 8 and the developmentelectrode 10. These 4 means may include, for example, a battery 36 orsimilar D. C. power source connected through a potentiometer 35 to oneor the other of the Xerographic plate or the development electrode. Inthe embodiment shown, it is connected to the development electrode.

Many variations of the apparatus will instantly be apparent. Thus, inthe placing of the insulating spacers 37 at each end of the assemblythere may be substituted therefor conductive spacers which will positionthe xerographic plate at the correct distance from the developmentelectrode 10 and serve as dust seals. With this change the ground on thepowder cloud supplying means, here shown connected to the metal jacket11 on tube 20, will ground the spacers 37 and the conductive backing 23of the xerographic plate when placed thereon. This eliminates thenecessity of a separate ground for conductive backing 23. Insulatingmeans 38 will suffice along with the insulating dust seals, not shown,along the sides of the assembly to keep the development electrode 10electrically insulated from the rest of the assembly.

In operation a xerographic plate is charged and exposed by any of themeans known to those skilled in the art to place thereon anelectrostatic charge pattern corresponding to a pattern of light andshadow to be reproduced. The image-bearing surface is then scanned witha vibrating probe electrometer, in the dark, to determine the maximumpotential on the image-bearing surface. The plate is then positioned onthe spacers 37 as shown in Fig. 2. Potentiometer 35 is then adjusted toplace on the development electrode 10 a potential approximately equal tothe highest potential found on the image-bearing surface. The gas supply18 and vacuum means 32 and drive means for reels 38 are now actuated.The gas supply 18 maintains enclosed chamber 27 at a pressure greaterthan atmospheric. This causes a stream of gas to pass into nozzle 39which entrains charcoal powder particles from ribbon 30 thereby creatinga suspension of powder particles in gas. The powder cloud is carriedalong by the air stream through tube 20 into manifold 14 where itexpands and passes through jets 13 into the entrance chamber 12. Thetube 20 has a restricted cross section so that the powder is inturbulent flow while passing therethrough and by repeated triboelectriccontact with the walls 9 thereof gives up electrons to the tube tobecome itself positively charged. The additional air flow from airsupply 18 through tube 19 enters the entrance chamber 12 through jets 31where the extra air mingles with the powder cloud. The thus augmentedcloud now enters the development zone 25 where it passes over thephotoconductive insulating layer 24 depositing electroscopic particlesin accordance with the charge pattern produced thereon by the action ofthe potential applied to the development electrode 10 on theelectrostatic latent image on the photoconductive layer 24. The powdercloud then leaves the development zone 25 through means 21 whereby it isfed into exhaust means 22. A slight negative pressure is kept on theexhaust means by suitable means 32. The powder passes through means 32into a collecting box 33 wherein the powder is separated from the gassuspension by a filter 34.

When operated in this means using soapstone for the tube liner 9, thenozzle 39 and the jets 13 a highly efficient positive charge wasimparted to the finely-divided charcoal powders. By this means there wasobtained a high quality, continuous tone, reversal print of the originalto be reproduced without any trouble due to agglomeration, unduedeposition in unwanted areas or objectional sparking or arcing.

In Figs. 3 and 4, there is shown a development apparatus according toanother embodiment of this invention.

According to this device one or more capillary or like ducts 20 feed toan image support member 10 which preferably has an extremely smoothupper surface extending in a horizontal plane. According to thepreferred embodiment of this invention, this support member desirably iselectrically grounded or may be held at some suitable bias potentialthus acting as a development electrode. The back and sides of thesupport member 10 have walls 44 with supporting ledges or shoulders 45which are adapted to receive and support an image-bearing member 8 withthe image-bearing surface extremely closely spaced in parallelrelationship to the smooth upper surface of the support member 10.According to the usual practices of Xerography, the image member 8comprises a conductive backing member 23 for a photo'- conductiveinsulating layer 24 on at least one surface thereof and when such animage-bearing member is used in conjunction with this support, thephotoconductive insulating layer 24 is faced toward member 10 therebydefining a development zone 25 and the conductive backing member 23 ispreferably electrically grounded. Capillary or duct 20 is preferablyconstructed of soapstone 9 and jacketed with metal 11 which is grounded.

In operation, the gas supply 18, such as an air pump, is activatedcausing the powder cloud generator 27 to supply a suspension offinely-divided charcoal in gas through duct 20 in turbulent flow,whereby the finelydivided charcoal repeatedly contacts the coapstonewalls 9 of duct 20 giving up electrons thereto and becoming itselfpositively charged.

Where the photoconductive insulating layer is zinc oxide in a binder,the electrostatic latent image thereon is composed of negativeelectrostatic charges. This device causes powder particles to depositthereon in faithful conformity with the image to be reproduced.

Fig. is a side elevation of a modification showing an alternativedeveloping zone. In this variation, there is a delta-shaped expansionchamber 12 where the powder cloud from the capillary or duct 20 impingesnormally on a deflecting surface which may, if desired, be an edge orborder portion of the image member 8 on image member support 10. In thismanner, the suspension of finelydivided charcoal in gas is directedoutwardly as illustrated by the arrows 42 and is caused to flowsubstantially straight across the surface of the member 8. Otherwise,the construction and operation of the apparatus is as shown in Figs. 3and 4.

Using soapstone for the tube liner 9 in the apparatus of Figs. 5 and 6over 100 reversal prints were made consecutively by placing a suitablebias on support as set forth in connection with Fig. 2. Even after thiscontinuous use, the pictures were all high quality, continuous tone,reversal prints without any trouble due to agglomeration, unduedeposition of charcoal in unwanted areas or objectionable sparking orarcing. The process was repeated using each of the following materialsfor liner 9: gray sandstone, quartzose sandstone of the argillaceoustype, pyrophyllite, serpentine, oligoclase, spodumene, halloysite,montmorillonite and bentonite. In each case a high degree of positivecharge was imparted to the particles of finely-divided charcoal makingpossible the obtaining of a high quality reversal print of theelectrostatic charge pattern originally occurring on photoconductiveselenium layers. While all of the materials named gave excellent resultsin positively charging finely-divided charcoal particles, soapstone,montmorillonite and bentonite were particularly outstanding in ease offabrication and handling.

I claim:

l. A process for electrically charging finely-divided charcoal to apositive polarity comprising entraining said charcoal in a stream of airand then passing said entrained charcoal in turbulent flow through anarrow conduit the interior walls of which consist of a mineral selectedfrom the group consisting of soapstone, montmorillonite, halloysite,bentonite, spodumene, red sandstone, serpentine, pyrophyllite,oligoclase, grey sandstone, and quartzose sandstone of the argillaceoustype.

2. A process according to claim 1 wherein the interior walls of theconduit consist of montmorillonite.

3. A process according to claim 1 wherein the interior walls of theconduit consist of soapstone.

4. A process according to claim 1 wherein the iriterior walls of the theconduit consist of bentonite.

5. An improved xerographic developing method wherein a positivelycharged suspension of finely-divided charcoal powder particles ispresented to an electrostatic latent image-bearing surface, said methodcomprising forming a gas suspension of said finely-divided charcoal,forcing said gas suspension through a capillary turbulence zone at arate of How therethrough su'fiicient to achieve turbulent flow incontradistinction to laminar flow whereby the suspended charcoalparticles are deagglomerated and are electrostatically charged to apositive polarity by contact with the walls of the capillary zone, saidwalls consisting of a mineral selected from the group consisting ofsoapstone, montmorillonite, halloysite, bentonite, spodumene, redsandstone, serpentine, pyrophyllite, oligoclase, grey sandstone, andquartzose sandstone of the argillaceous type and presenting thesuspension of said positively charged charcoal particles from saidturbulent zone to the electrostatic latent image-bearing surface.

6. A process according to claim 5 wherein the walls of the capillaryzone consist of montmorillonite.

7. A process according to claim 5 wherein the walls of the capillaryzone consist of soapstone.

8. A process according to claim 5 wherein the walls of the capillaryzone consist of bentonite.

9. Apparatus for making visible an electrostatic latent image on animage-bearing member comprising means to create a suspension of cloudparticles in gas, support means for an image-bearing member' having anelectrostatic latent irnage thereon, an elongated zone of turbulent flowcomprising a tubular member of restrictive cross section between saidmeans to create a suspension of powder particles in gas and saidsupport, and means to flow said suspension into said tubular memberwhereby the gas suspension of particles flowing through the tubularmember from said means to create a suspension of powder particles in gasis caused to be in turbulent flow thus bringing the suspended particlesinto repeated triboelectrically charging contact with the walls of saidmember during passage therethrough, the walls of said member being amineral selected from the group consisting of soapstone,montmorillonite, halloysite, bentonite, spodumene, red sandstone,serpentine, pyrophyllite, oligoclase, gray sandstone, and quartzonesandstone of the argillaceous type.

10. Apparatus for making visible an electrostatic latent image on animage-bearing member comprising means to create a suspension of powderparticles in gas, support means for an image-bearing member having anelectrostatic latent image thereon, at least one capillary tube betweensaid means to create a suspension of powder particles in gas and saidsupport, and means to flow said suspension into said tube whereby thegas suspension of particles flowing through said tube from said means tocreate a suspension of powder particles in gas is caused to be inturbulent flow thus bringing the suspended particles into repeatedtriboelectrically charging contact with the walls of said tube duringpassage therethrough, said walls being a mineral selected from the groupconsisting of soapstone, montmorillonite, halloysite, bentonite,spodumene, red sandstone, serpentine, pyrophyllite, oligoclase, graysandstone, and quartzose sandstone of the argillaceous type.

11. Apparatus for making visible an electrostatic latent image on animage-bearing member comprising a source of gas under pressure feedinginto a cloud generator, said cloud generator being adapted to create asuspension of powder particles in gas suspension, means for supportingan image-bearing member having an electrostatic latent image thereon andat least one capillary tube between ,said cloud generator and saidsupport means whereby the gas suspension of particles flowing into thetube from the cloud generator is caused to be in turbulent flow thusbringing the suspended particles into repeated triboelectricallycharging contact with the walls of saidtube during passage therethrough,the walls of said tube being a mineral selected from the groupconsisting of soapstone, montmorillonite, halloysite, bentonite,spodumene, red sandstone, serpentine, pyrophyllite, oligoclase, greysandstone, and quartzose sandstone of the argillaceous type, said tubefurther being adapted to convey the suspension of particles into thespace between the image-bearing member and a closely spaced conductiveelectrode.

2, References Cited in the file of this patent UNITED STATES PATENTSPhillips June 21,1955 Landn'gan et. a1 Nov. 29, 1955 OTHER REFERENCESShaw! Experiments on Tribo-Electricity, Proc. Roy. Soc.,-19l7, A94, pp.16 21 and table opposite p. 32,

Fraas et al.: Electrostatic Separation of Solids, Ind. and Eng. Chem.,v01. 32, No. 5, pp. 600-604.

Young et. al.: Electrofax, Reprint from R. C. A. Review, December'1954,vol. XV, No. 4, p. 479.

5. AN IMPROVED XEROGRAPHIC DEVELOPING METHOD WHEREIN A POSITIVELYCHARGED SUSPENSION OF FINELY-DIVIDED CHARCOAL POWDER PARTICLES ISPRESENTED TO AN ELECTROSTATIC LATENT IMAGE-BEARING SURFACE, SAID METHODCOMPRISING FORMING A GAS SUSPENSION OF SAID FINELY-DIVIDED CHARCOAL,FORCING SAID GAS SUSPENSION THROUGH A CAPILLARY TURBULENCE ZONE AT ARATE OF FLOW THERETHROUGH SUFFICIENT TO ACHIEVE TURBULENT FLOW INCONTRADISTINCTION TO LAMINAR FLOW WHEREBY THE SUSPENDED CHARCOALPARTICLES ARE DEAGGLOMERATED AND ARE ELECTROSTATICALLY CHARGED TO APOSITIVE POLARITY BY CONTACT WITH THE WALLS OF THE CAPILLARY ZONE, SAIDWALLS CONSISTING OF A MINERAL SELECTED FROM THE GROUP CONSISTING OFSOAPSTONE, MONTMORILLONITE, HALLOYSITE, BENTONITE, SPODUMENE, REDSANDSTONE, SERPENTINE, PYROPHYLLITE, OLIGOCLASE, GREY SANDSTONE, ANDQUARTZOSE SANDSTONE OF THE ARGILLACEOUS TYPE AND PRESENTING THESUSPENSION OF SAID POSITIVELY CHARGED CHARCOAL PARTICLES FROM SAIDTURBULENT ZONE TO THE ELECTROSTATIC LATENT IMAGE-BEARING SURFACE.